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Service compatibility is more than type compatibility

Connected Services Framework
Procedural and object-oriented designs typically equate type compatibility with semantic compatibility. Service orientation provides a richer model for determining compatibility. Structural compatibility is based on the contract (WSDL and optionally BPEL4WS) and the schema (XSD) and can be validated. Moreover, the advent of WS-Policy provides for additional, automated analysis of the service-assurance compatibility between services. This is done based on explicit assertions of capabilities and requirements in the form of WS-Policy statements.

Using WS-Policy, services describe their service-assurance capabilities and requirements in the form of a computer-readable policy expression containing combinations of assertions. This allows services to select each other based on "how" or "with what quality" they deliver their contracts.

Each policy assertion is identified by a stable and globally-unique name, which has a meaning that is consistent in time and space no matter which service the assertion is applied to. A policy assertion can also have parameters that qualify the exact interpretation of the assertion.

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